Re: pmc(3): when are the counters updated?
Le 11 mars 2012 à 16:45, Vitaly Magerya a écrit :
Hi, folks. I'm trying to use pmc(3) to analyze code fragments, and
I've run into strange behavior: the counter values returned by
pmc_read(3) sometimes show no increment between readings, but are
updated a second later; even if the PMC in question was stopped
before.
Here's a test program:
#include sys/types.h
#include pmc.h
#include stdio.h
#include unistd.h
int main() {
pmc_id_t pmcid;
pmc_value_t value;
int i;
pmc_init();
pmc_allocate(instructions, PMC_MODE_TC, 0, PMC_CPU_ANY, pmcid);
pmc_start(pmcid);
for (i = 0; i 500; i++);
pmc_stop(pmcid);
pmc_read(pmcid, value);
printf(first reading: %lu\n, (unsigned long)value);
sleep(1);
pmc_read(pmcid, value);
printf(second reading: %lu\n, (unsigned long)value);
}
It's output on my system (FreeBSD 8.2 amd64, an Intel Atom processor)
is something like this:
first reading: 0
second reading: 15090110
I don't really like both numbers; I expect the first reading not
to be zero (there obviously are instructions between pmc_start and
pmc_stop), and I expect the second reading not to differ from the
first, as the PMC was stopped before both of them, so it's value
should not change.
So, what's going on here? Is this the intended behavior, or can it
be changed? And how do I get accurate readings?
If i remember well:
The current code will get real HW PMC if the PMC is running and attached to
owner.
The first case is not true in your code so you get the saved value which is
updated at
process switch out time.
I think you can do:
pmc_read
pmc_stop.
(BTW, is this the right list for such questions?)
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Re: pmc(3): when are the counters updated?
Fabien Thomas wrote: So, what's going on here? Is this the intended behavior, or can it be changed? And how do I get accurate readings? If i remember well: The current code will get real HW PMC if the PMC is running and attached to owner. The first case is not true in your code so you get the saved value which is updated at process switch out time. I think you can do: pmc_read pmc_stop. Changing the code to this: [...] pmc_read(pmcid, value); pmc_stop(pmcid); printf(reading #1: %lu\n, (unsigned long)value); pmc_read(pmcid, value); printf(reading #2: %lu\n, (unsigned long)value); sleep(1); pmc_read(pmcid, value); printf(reading #3: %lu\n, (unsigned long)value); [...] Gives me this output: reading #1: 15039441 reading #2: 0 reading #3: 15084037 So, it seems that you are correct; pmc_read reports current values if the PMC is not stopped, after which it's updated on context switches. ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
Re: pmc(3): when are the counters updated?
2012/3/12 Vitaly Magerya vmage...@gmail.com Fabien Thomas wrote: So, what's going on here? Is this the intended behavior, or can it be changed? And how do I get accurate readings? If i remember well: The current code will get real HW PMC if the PMC is running and attached to owner. The first case is not true in your code so you get the saved value which is updated at process switch out time. I think you can do: pmc_read pmc_stop. Changing the code to this: [...] pmc_read(pmcid, value); pmc_stop(pmcid); printf(reading #1: %lu\n, (unsigned long)value); pmc_read(pmcid, value); printf(reading #2: %lu\n, (unsigned long)value); sleep(1); pmc_read(pmcid, value); printf(reading #3: %lu\n, (unsigned long)value); [...] Gives me this output: reading #1: 15039441 reading #2: 0 reading #3: 15084037 So, it seems that you are correct; pmc_read reports current values if the PMC is not stopped, after which it's updated on context switches. Hello, I've used the PMC library in the past, and I think that you can't stop the counters if you want to read them. By having them started, you can read their value anywhere in the code, not only in context switches. I suggest you changing the code to something like pmc_read(..); do_some_calculation(..); pmc_read(..); /* difference between reads is devoted to do_some_calculation */ and when you're about to finalize the application, issue pmc_stop. Please, also note, sleep(1) yields the process, so counting it (even if it's 1 second or more) will probably result in very low counter values. Regards -- Fry: You can see how I lived before I met you. Bender: You lived before you met me?! Fry: Yeah, lots of people did. Bender: Really?! ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
pmc(3): when are the counters updated?
Hi, folks. I'm trying to use pmc(3) to analyze code fragments, and
I've run into strange behavior: the counter values returned by
pmc_read(3) sometimes show no increment between readings, but are
updated a second later; even if the PMC in question was stopped
before.
Here's a test program:
#include sys/types.h
#include pmc.h
#include stdio.h
#include unistd.h
int main() {
pmc_id_t pmcid;
pmc_value_t value;
int i;
pmc_init();
pmc_allocate(instructions, PMC_MODE_TC, 0, PMC_CPU_ANY, pmcid);
pmc_start(pmcid);
for (i = 0; i 500; i++);
pmc_stop(pmcid);
pmc_read(pmcid, value);
printf(first reading: %lu\n, (unsigned long)value);
sleep(1);
pmc_read(pmcid, value);
printf(second reading: %lu\n, (unsigned long)value);
}
It's output on my system (FreeBSD 8.2 amd64, an Intel Atom processor)
is something like this:
first reading: 0
second reading: 15090110
I don't really like both numbers; I expect the first reading not
to be zero (there obviously are instructions between pmc_start and
pmc_stop), and I expect the second reading not to differ from the
first, as the PMC was stopped before both of them, so it's value
should not change.
So, what's going on here? Is this the intended behavior, or can it
be changed? And how do I get accurate readings?
(BTW, is this the right list for such questions?)
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